The use of sound waves to test train wheels in order to detect defects, such as cracks, is well known. Typically a hammer-like element, analogous to a bell clapper, is used to strike the moving train wheel. As a result of the impact, sound waves, hereafter simply referred to as sound, is produced. Pick up devices pick up the sound and convert it to electrical signals, which are then transmitted to an analysing device e.g. a computer.
Various devices, often referred to as exciters, are presently used to strike train wheels. One of these devices is of the treadle type. The passing train wheel mechanically activates a treadle which then drives the hammer head to strike the wheel. The treadle type exciter is not very reliable. It is only useable with a very slow moving train up to 1 or 2 miles per hour (mph) and the impact force which it produces is not constant, resulting in difficulties in the analysis of the produced sounds. Another known exciter employs a solenoid to drive a hammer. This exciter also suffers from significant limitations. It is useable with trains moving at not more than about 10 mph. Also, it requires high electrical current, on the order of 100 amps to drive the hammer.